Abstract

As the responsibility of R&D spending in the oil field shifts to service companies, many new technologies are introduced as tools. Combining multiple tools or processes can present new and exciting answers to old intervention challenges.

This paper describes coiled tubing operations using a newly developed straddling system on two particular wells that were producing from several perforated intervals. The key to both operations was to first isolate each zone, perform injectivity testing at varying flow rates, and then to stimulate each zone at a rate below fracture pressure. The initial challenge was the large number of zones - 16 zones in the first well and 14 zones in the second well - leading to the requirement to have multisettable technology in order to complete the individual zonal treatments in as few runs into the well as possible. The second challenge entailed the need for accurate depth control to ensure the zones were correctly straddled to allow for optimum treatment. If the packer elements were not set in the exact place, the treatment could be misplaced. A wireless casing collar locator (CCL) with pump-through capability, contained as standard within the tool, was used, delivering accurate, real-time depth readings without any added limitations or further runs in the well.

Inflatable technology was preferred over the conventional mechanical-cup isolation because of subsidence in the area and casing integrity concerns. Based on laboratory testing of the two reservoir types, two chemical systems were selected for the stimulation treatment; one was hydrofluoric clay acid-based and another xylene-based. Both chemicals are known to be chemically aggressive to the conventional rubber seals and elastomers found in inflatable technology. It was imperative that the inflatable could withstand these chemicals for the treatment to succeed. Multiple setting of the tool was completed by using flow pattern recognition devices in the tool. Sending individual pumping sequences from the surface through the coiled tubing to the tool accessed the different modes alternating between inflation, injection, circulation, and deflation mode. A feedback signal from the tool was received at the surface as a confirmation that the tool had correctly actuated to the desired mode.

A real-time correlation log was created; the inflatable packer was multiset on depth, injectivity testing took place and the clay acid- and xylene-based stimulations were pumped into all zones. The multiple technologies were combined in an innovative application that met all objectives of the operation.

Introduction

Subsidence in the region of Lost Hills field has been a major concern for several years. More than a risk, varying degrees of subsidence have been affecting the casing integrity of a large number of wells in the area.

The Lost Hills reservoirs characteristically possess high porosity and low permeability, typically 40% porosity, 0.01 to 1 md permeability, with a pay zone of approximately 1,000 ft. The hydrocarbon potential in Lost Hills field is very large but because of ineffective production methods, the field's full potential has not been attained.

It was believed that the natural production decline had subsequently been further affected by near-wellbore formation damage (fines, scales, water block, paraffin, asphaltenes, wax, etc.) and that this had resulted in serious production problems across the field. In order to remedy these types of formation damage, both chemical and mechanical approaches had been previously attempted. The limitations of simple bullheaded matrix treatments are that the upper producing zones, into which the majority of the pumped volume enters, are prone to unwanted fracturing. This approach, unsurprisingly, results in very poor fluid distribution with virtually no treatment fluid entering the lower zones. Damage to casing resulting from subsidence limits the ability to run full wellbore tools downhole to lower depths. These lower zones are commonly the richest pay zones. For that reason, inflatable tools soon became an applicable option. In the past, the casing collapse had been so adverse that even slim bottomhole assembly tools were not deployable through such casing restrictions. In these cases, high-expansion-ratio inflatable tools have become a necessity in order to fulfill the intervention objectives and reach the lower zones.

The two wells intervened in this project, Section 33 Well 3–9F and Section 5 Well 12–2D, did not display the extreme subsidence-related casing damage mentioned above, although the risk was still present. Those wells displayed symptoms of formation damage and were selected as optimum candidates because lower zones could be accessed using a slim bottomhole assembly, thus involving lower retrieval risks.

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